This invention relates in general to the physical separation of constituents of a mixture of particles, more particularly to improvements in methods and means for separating constituents of a mixture of ultra-fine particles which do not exceed about 100 microns in size. The invention is applicable to a wide variety of physical mixtures, as well as to the beneficiation of ores. It has been found to be particularly useful in the separation of impurities from coal, i.e.: coal beneficiation, and to prepare ultra-fine pulverized coal for burning in boilers designed for burning fuel oil or natural gas.
The constituents of coal which are considered to be "impurities" include those containing sulfur and some minerals which form non-combustible ash. Ash-forming constituents coat, foul and drastically reduce the efficience of heat transfer in boilers, in addition to polluting the environment. Sulfur-bearing constituents contribute to environmental pollution, one form of such pollution being commonly referred to as "acid rain". As found in its natural state, coal contains varying proportions of these impurities, the proportions in any one deposit depending on the geological history of that deposit.
Coal beneficiation begins with a process of crushing, pulverizing, or comminuting coal, to break pieces of coal down to particles of smaller and smaller sizes, which frees the constituents from one another and thereby enables them to be separated. Eventually, this process yields particle sizes so small that the cost and difficulty of handling the product become formidable barriers to further progress. Finely-comminuted coal particles can be confined in a liquid slurry for further treatment, but that approach requires the use of water or other liquid, which adds to the cost and complexity of the separation process and therefore is not economically or logistically desirable on a commercial scale. Dry separation processes, among which processes for separating charged particles in an electric field in a gaseous medium (e.g.: roll-collectors with "corona" charging) are highly desirable, suffer the limitation that coal comminuted so fine (smaller than 37 microns or 400 mesh) as to be like a powder, blows around in a dust-like cloud of fine particles and is very difficult to deposit on a rotating roll-collector; it contributes a potential source of explosion which can be initiated by a spark in the apparatus. Yet, the finer the coal is comminuted the greater is the portion of the impurity constituents that can be physically freed for eventual separation from the coal.
Similar problems are encountered in the beneficiation of phosphate ores, which are mined in a matrix comprised of pieces of phosphate rock and silica admixed in a clay-like material known as "slimes." The matrix material must be disintegrated as much as possible in order to efficiently recover phosphate rock. In the process, significant quantities of ultrafine particles (slimes) are produced. Prior art beneficiation methods cannot separate ultrafine particles efficiently or effectively.
This invention teaches new methods and means for electrically charging and finely comminuting constituents of coal and other ores to powder-like ultra-fine particles sizes (e.g.: smaller than 100 microns), and for electrically charging a mixture which includes ultra-fine particles, so as to enable particles of impurities and particles of coal, phosphate or other desired component, or constituents of any such mixture, to be separated from each other in an electric field in particle size ranges smaller than have heretofore been separable on a commercial scale in an electric field in a gaseous medium.